1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting device and a method for fabricating the same, and more particularly, to a light emitting device, which is configured to prevent or minimize exposure of a protective metal layer.
2. Discussion of the Background
In general, since Group-III-element nitrides, such as GaN and AlN, have excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as a material for light emitting devices in blue and ultraviolet regions. Particularly, blue and green light emitting devices using GaN are used in various applications such as large-sized full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems and optical communications.
Such a Group-III-element nitride, particularly GaN, is grown on a heterogeneous substrate with a similar crystal structure through a metal organic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy (MBE) method, or the like because it is difficult to form a homogeneous substrate on which GaN is grown. A sapphire substrate with a hexagonal system structure is mainly used as the heterogeneous substrate. However, since sapphire is an electrical nonconductor, it limits the structure of a light emitting diode (LED). Since sapphire is mechanically and chemically very stable, it is difficult to perform processing, such as cutting, shaping and the like, and the thermal conductivity of the sapphire is low. Accordingly, studies have been recently conducted to develop a method in which nitride semiconductor layers are grown on a heterogeneous substrate such as sapphire, and the heterogeneous substrate is separated from the nitride semiconductor layers, thereby fabricating a vertical type LED.
FIG. 1 is a sectional view illustrating a conventional vertical type LED.
Referring to FIG. 1, the vertical type LED includes a conductive substrate 31. Compound semiconductor layers including an N-type semiconductor layer 15, an active layer 17 and a P-type semiconductor layer 19 are formed on the conductive substrate 31. Also, a metal reflection layer 23, a protective metal layer 25 and an adhesive layer 27 are interposed between the conductive substrate 31 and the P-type semiconductor layer 19.
Generally, the compound semiconductor layers are grown on a sacrificial substrate (not shown), such as a sapphire substrate, using a MOCVD method or the like. Then, the metal reflection layer 23, the protective metal layer 25, and the adhesive layer 27 are formed on the compound semiconductor layers, and the conductive substrate 31 adheres thereto. Subsequently, the sacrificial substrate is separated from the compound semiconductor layers using a laser lift-off method or the like, and the N-type semiconductor layer 15 is exposed. Thereafter, the compound semiconductor layers are separated into respective light emitting cell regions on the conductive substrate 31 through an etching process. Then, an electrode pad 33 is formed on the N-type semiconductor layer 15 in each of the light emitting cell regions, and individual devices are separated by dicing the conductive substrate 31 for each of the light emitting cell regions. Accordingly, the conductive substrate 31 having an excellent heat radiation performance is employed, thereby improving light emitting efficiency of an LED and providing the LED of FIG. 1 having a vertical structure.
However, in cases when the vertical type LED uses the conductive substrate, a dry etching process is typically performed to separate respective cells from each other when fabricating them. Since such an etching process is to separate the device itself into the respective cells, it is performed to have a depth (2 μm or more) deeper than a mesa etching process. Therefore, the etching process is performed to have a depth deeper than the actually etched depth in order to remove residue partially remaining at the exposed portion after the etching.
In the etching process, the protective metal layer 25 that protects the metal reflection layer 23 is etched, and etched residue thereof adhere to sides of each of the cells. The residue adhering to each cell electrically connect the N-type semiconductor layer 15 and the P-type semiconductor layer 19 to each other, thereby causing a short circuit. Such residue that may be produced in the etching process should be removed through a wet etching process. However, it is difficult to remove the residue because metals such as W, Pt, or Ni used as the protective metal layer 25 are generally not removed even through the wet etching process.